Radio speaker control structure and circuit for use with automatic tuning devices for radio receivers



Aug. 22, 1939. w DUNN 2,170,261 RADIO SPEAKER CONTROL STRUCTURE ANDCIRCUIT FOR USE WITH AUTOMATIC TUNING DEVICES FOR RADIO RECEIVERS FiledJan. 6, 1938 2 Sheets-Sheet l iinw Aug. 22, 1939. DUNN 2,170,261

RADIO SPEAKER CONTROL STRUCTURE AND CIRCUIT FOR USE WITH AUTOMATICTUNING DEVICES FOR RADIO RECEIVERS Filed Jan. 6, 1938 2 Sheets-Sheet 2 v/a 4 cfizz ezz/oz 5/ I III; 7 1220272 Patented Aug. 22, 1939 RADIOSPEAKER, CONTROL STRUCTURE AND CIRCUIT FOR USE WITH AUTOMATIC TUNINGDEVICES FOR RADIO RECEIVERS William L. Dunn, Chicago, 111., assignor toBelmont Radio Corporation, Chicago, 111., a corporation of IllinoisApplication January 6, 1938, Serial No. 183,577

13 Claims.

My invention relates in general to radio control apparatus and more inparticular to means for selectively rendering the loud speakerof theradio receiver inoperative when using the con-- trol apparatus to tunethe radio receiver and rendering the loud speaker operative whilepresetting the adjusting portions of the control aparatus tosubsequently provide for tuning the radio receiver to predeterminedcontrol positions. The invention is particularly applied to so-calledautomatic control apparatus where the frequency controlling means ofsuch apparatus is automatically operated to a predetermined controlposition or broadcast station setting. One commercial application of theinvention is on control apparatus for a radio receiver including camsand levers such as is shown. in the copending application of AlexanderW. Plensler Serial No. 1'70,(l70. It is understood, of course, that theapplication of this invention is not limited to this particular type ofmechanism.

In any tuning apparatus. for radio receivers where the actual frequencycontrol is automatically accomplished by pressing a'lever, a button orany of the other various means now employed, the operator normally makesno change in the speaker volume at the same time the automatic tuning isbeing accomplished. As a result, there is objectionable noise in thespeaker as the control apparatus passes through. various frequencies inbeing tuned to the desired predetermined station. This noise is not onlyobjectionable to the ear, but in receivers having high power output, thepeak voltages created by rapidly tuning through powerful stations islikely to damage the loud speaker of the receiver.

pensive and not adapted to all types of automatic control apparatus. Thepresent invention util-' izes the elements of the control apparatusitself for silencing the loud speaker during automatic tuning andlikewise provides. for temporary operation of the speaker in thecomplete tuning operation merely while presetting adjustments in theautomatic control apparatus are being accom- This is an old problem. intheautomatic tuning art, but the solutions to'thisj problem heretoforeprovided are ordinarily ex- It is an object of my invention to provide asimplified and improved speaker control system for use in connectionwith an automatic tuning device for a radio receiver.

A further object is to provide such a device which may readily adaptitself to automatic tuning structure and in fact utilize some of theelements of such structure for the operation of the system so that aminimum number of parts will be required.

A still further object is the provision of such speaker controlapparatus particularly adapted to automatic tuning apparatus wherein thetuning apparatus includes movable levers so arranged that the end pointof travel of the levers in accomplishing automatic tuning is constantregardless of the arrangement of the movable control elementscorresponding to predetermined station settings themselves.

One of the features of my invention is the provision of mechanism foraccomplishing silent automatic tuning in a radio receiver.

A further feature is the provision of a speaker control system whichrenders the radio speaker non-operative during automatic tuning, yetpermits the normal operation of said speaker while one or more of thetuning control members is adjusted to a predetermined station setting.

Other objects and features of my invention will be apparent from thefollowing description taken with the drawings, in which:

Fig. l is a schematic illustration of some of the elements of anautomatic tuning device using a cam and lever structure, with thecircuit for the speaker control system of my invention similarlyillustrated in connection with such mechanism.

Fig. 2 is a fragmentary View partly in section of the adjusting knobillustrated at the righthand side of Fig. 1 together with the switch inconnection therewith.

Fig. 3 is a more extensive schematic diagram of the circuit and elementsin the speaker control system of my invention, showing a differentmuting connection with the control apparatus.

Fig. 4 is a fragmentary end elevation of adjusting mechanism. and switchcorresponding to that of Figs. 1 and 2, yet modified therefrom.

Fig. 5 is a fragmentary sectional view on the line 55 of Fig. 4.

Fig. 6 is a fragmentary front elevation partly in section of a modifiedembodiment of my invention applied to cam and lever tuning mechanism.

Fig. 7 is a schematic diagram of the embodiment of Fig. 6 on the line.1-4 of Fig. 6, incombination with the circuit; diagram for thisembodiment.

In the embodiment of Fig. 1 a tuning condenser is illustrated at itsecured to a tuning shaft ll, supported in frame plates l2 and i3. Frameplate 83 is broken away in Fig. l merely to prevent confusion with theswitch in connection with the adjusting knob as will be hereinafter morefully described, but in fact extends upwardly to carry the shaft I i asseen in Fig. 2. The rotors in the condenser it] are rotated uponrotation of the shaft II and the latter rotation is accomplished bymoving a cam l4 by means of a pivoted lever IS. The cam M is normallystationary with respect to the shaft i I, but may be angularly adjustedrelative to said shaft to change the station setting corresponding tosuch cam. This adjustment may be accomplished, as is more fullydescribed in the above noted Plensler application, by means of a tuningand adjusting knob ll carried on the end of the shaft H as shown moreparticularly in Fig. 2. Although this specific structure is not afeature of the present invention, Fig. 2 might be described briefly asillustrating the knob i! keyed to a sleeve E8 on the rotary shaft H. Thecams M are normally retained stationary with respect to the shaft H bymeans of friction washers IS on each side thereof and slidable sleeves2! bearing against the washers 19 so that upon threadable adjustment ofthe cap screw 22, sleeves i8 and 2| press the cam and washer assembly ina left-hand direction as shown in Fig. 2 to bear against a rigid member(not shown) at the lefthand end of the cam assembly on shaft H. Assleeve I8 slides in a left-hand direction the knob H is carriedtherewith to change its axial position and inturn act upon the switcharms of the demuting switch D as will be more fully describedhereinafter.

As to the speaker control structure of my invention including thedemuting switch D, and as illustrated in Fig. 1, an electricalconnection 23 is provided between the metal cam I4 and one side as 24 ofthe speaker circuit, including the output transformer 26, the voice coil21, and field coil 28, and the speaker diaphragm itself is illustrateddiagrammatically at 29. In Fig. l, the connection to the cam I4 isillustrated as leading directly to such cam, but it is understood thatthis may be accomplished by a stationary brush bearing against the shaftI l which is electrically common to the cam I4, or by a Wire connectedto the end plate 52 which likewise is electrically common to the shaft Hand cam M. A pivot rod 3! is mounted in the end plates l2 and I3 andinsulated therefrom by means of fibre bushings 32, or the like, and suchpivot rod 31 supports one or more tuning levers it corresponding to thecam members Hi. The rod 3i and lever 16 are each metal and an electricalconnection is made from such rod 3! through the switch D by means ofleads of 33 and 34 to wire 36 forming the opposite side of the speakercircuit from the wire 24.

It is apparent that connecting the wire 24 directly to the wire 36 wouldshort-circuit the speaker circuit or, in other words, shunt out thevoice coil 21. This short-circuiting or shunting out of the voice coil27 is automatically accomplished when the lever it is depressed in theautomatic tuning operation. The shunting circuit is from wire 24 throughlead 23, through cam l4, through lever l6 and its pivot rod 3!, throughwire 33, through the normally closed switch D and through wire 34 towire 36. It follows that the speaker is muted during any automatictuning operation, the muting becoming effective as soon as the leverlt'engages the cam 54 and ceasing as soon as the automatic tuning iscomplete and the lever it departs from the cam i i.

Presetting the cams In presetting the cam M with respect to the tuningshaft it so that operation of the lever [B will turn the tuning shaft toa predetermined position in order to tune the radio receiver to aparticular frequency, the lever it is held in its fully actuatedposition as shown in dotted lines in Fig. '7, to hold the cam I4 in theposition to which it will always be thrust by the lever l6, and theshaft H is loosened from the cam 54 by reducing the frictional pressureon the cam so that the shaft may be tuned to the frequency which theuser desires to have correspond to this actuated position of cam I i.According to the muting operation as so far described, the contact ofthe lever it with the cam it would shunt out the voice coil 2? with theresult that it would be impossible to tune the set by ear in order toselect a particular station or obtain peak tuning for that station withrespect to the actuated position of the cam it. This would be anundesirable effect. In other words, it is desired that the actuation ofthe lever It should mute the speaker during automatic tuning operationsbut should not mute the speaker during actuation of this lever for thepurpose of presetting the cams i i on the shaft it. Of course, theactual presetting itself is accomplished when the lever is in camholding position as previously noted. It is to provide this differentialmuting effect that the switch D is provided, the function of this switchbeing to nullify the muting circuit during presetting of the cam Hi.This switch could for convenience be called a demuting switch. Brieflystated, the switch D is normally closed to render the muting circuiteffective when lever it engages cam id, but is automatically opened byoperation of the knob if in the presetting operation so as to render themuting circuit ineffective during that operation.

The details of the actuation of switch D are seen in Fig. 2, When thescrew 22 is loosened in order to relieve the pressure from sleeves l8and 2 l spring pressure from one or more spring washers W in the camassembly slides the sleeve E8 to the right and with it the knob H. Thecontact arms themselves in the switch D could provide this slidingpressure, and with the knob in the extended position, the camsthemselves will readily loosen with respect to the shaft ii when thepresetting operation is undertaken. The knob I! engages contact spring353 of switch D and normally holds it in engagement with contact spring39 of switch I) to close the muting circuit. When the screw 22 isloosened, however, the handle El moves to the right. This permits springcontact 38 to spring away from contact 39 and open the muting circuit,thus rendering the muting circuit ineffective. The loud speaker 29 maynow function, and while cam 14 is held in its actuated position by leverit, shaft I i may be turned by knob I! to tune in the desired stationaccurately. This presets the cam M with respect to the shaft II. Thespring washer W will act to hold this preset adjustment even though thelever I6 is released, since there will normally be several such cams i ieach having a lever l6 and only one lever should be actuated at once.When all of the cams l4} have been preset, the cap screw 22 may betightened, thus locking the cams |4 permanently in their preset"positions and drawing the knob I! in against the contact spring 38 toclose the switch D. This restores the muting circuit to operativecondition so that it will mute the speaker when the lever I6 is nextoperated.

Alternative muting connections There are various places in which themuting circuit may be connected instead of being con nected across thevoice coil 21 as seen in Fig, 1. In Fig. 3, for example, it has beenshown in full lines as being connected by the wires 23a. and G latoinput lead 4| and ground connection 42. The effectof this istoshort-circuit the input of the tube 43 to ground so that this tube isrendered inoperative and therefore no impulses are trans-v mittedthrough the remainder of the set through the speaker 29.

In dotted lines in this same figure the muting circuit is indicated asconnected at an alternative position by wires 23b and 34b in. the samemanner to the input connection of output tube 44 1 and to ground so asto short-circuit the signal impulses at this point. From these examplesit will be apparent that the muting circuit may be connected at avariety of. points, the only requirement being that it be so connectedasto directly I or indirectly mute the speaker 29.

Alternating demuting switch With different forms of presettingarrangements different switches rnay be necessary for nullifying themuting circuit during presetting.

One other presetting construction is shown in Figs. 4 and 5, in whichthe cap screw 22 is replaced by a threaded collar 46 to which a toothedWheel 4'! is secured, the wheel 4? normally being free to rotate withthe shaft I la. When it is desired to loosen the cam 54, a lever 48 isdepressed so as to cause a dog 49 on the end thereof to engage the teethof wheel 4'l, thus locking the wheel. The shaft 5 la may then be turnedin any suitable manner as by knob 5| thereon and since the collar 46 andWheel 41 are now restrained from turning by the dog 49, the wheel 41will unscrew with respect to the shaft Ila along the threaded portionthereof, thus releasing the presi sure on the friction washers l9 andloosening the cam l4 sufficiently to permit the presetting adjustmentthereof with respect to the shaft i la as previously described.

- The lever 48 is normally retained in a retracted position by asuitable spring 52. In such retracted position it engages the springcontact 38 of. switch D, pressing it against the springcontact 39 andthus closing the switch D to put the muting circuit in an operativecondition ready to mute the 'speaker when one of the levers I5 isoperated.

Restricted muting In Figs. 6 and 7 a muting arrangement has beenillustrated which automatically cancels the muting efiect at the end ofeach tuning operation as the tuning lever reaches its fully actuatedposition. This makes a separately actuated demuting switch unnecessarysince the presetting of the cam always takes place with the lever fullyactuated to hold the cam in its actuated position. The automaticcancelling of the muting at the end of each tuning operation has theadditional advantage of restoring the set to operation even though aninexperienced user should hold the tuning lever depressed.

In this form of the invention one muting connection 6| extends to a lug62 carried by a screw 63 which is supported by but insulated from theframe member l3. Normally engaging the screw 53 is a demuting bar 64which is pivotally carried by the pivot bar 3| which also carries thetuning lever B. The demuting bar 64 is normally urged upwardly intoengagement with demuting screw 63 by a suitable spring 66. When thelever I6 is depressed, however, it will, near the end of its stroke,strike demuting bar 54 and depress it so that its contact with demutingscrew 63 is broken, its depressed position being shown in dotted linesin Fig. '7. It should be noted that the spring 66 must be insulatedeither from the demuting bar 64 or from the frame member by which it issupported, and in Fig. 7, it has been shown as insulate-d from a crossbar 61 by a rubber sleeve 68 to which it is secured.

The cross bar 61 and rubber sleeve 68 also serve as an insulated stopfor the'lever l6 in its normal position, any suitable means beingprovided for biasing the lever to this position. Of course, it isunderstood that such biasing means and a suitable insulated stop couldalso be provided for the tuning lever l6 in Fig. 1. It may be mentionedthat bars are used for the elements 64 and 61 rather than short lugsbecause there are usually five or more tuning levers I6, and theelongated bars may serve them all.

In Fig. '7 the muting circuit which is closed when lever I6 strikes caml4 extends from one side of voice coil 2'! through lug 62, through screw63, through demuting bar 64 to pivot bar 34 and from this bar throughlever l5, cam 54, shaft H and frame plate It to ground. Since the otherside of voice coil 21 is also connected to ground, this muting circuitshort-circuits the voice coil 21.

The .dotted line 6| in Fig. '7 indicates an alternative connection forthe muting lead 6| similar to the connection 21 a or 231) of Fig. 3.With this connection the muting circuit would. mute the speakerindirectly by short-circuiting the input side of the tube 44.

The screw 63 is so positioned as to hold the demuting bar 64 out ofcontact with the tuning lever l6 until the latter is approximately atthe end of its stroke, perhaps of an inch from the end of its stroke. Atthis time the lever It breaks the contact between the demuting bar 84and demuting screw 63, and hence opens the muting circuit and renders itineffective so that as the cam I4 reaches the position desired, thespeaker will again become operative and thedesired program will beheard, or if it is desired to change the position'of cam It on shaftthis may be accomplished by turning the shaft with a suitable knob, suchas the knob ll of Fig. 1, while holding the lever I6 depressed. Theshaft may be turned to bring the condenser ID to peak tuning ofthedesired station, the proper setting being determined by ear becausethe loud speaker is functioning. 7

Of course, in any receiving set provided with a visual tuning indicatorwhich shows peak tun ing visually, the demuting switch D in Figs. 1 and4 and the corresponding demuting switch formed by demuting bar 6%andscrew 63 may be omitted, such demuting switches being necessary onlyto permit tuning by ear during presetting of the cam l4 on the shaft ii.Even with such visual tuning indicators, however, the arrangement ofFigs. 6 and 7 is preferred since it restores the receiving set tooperation immediately upon completion of the tuning, even if the tuninglever be held depressed.

From the foregoing it is seen that an extremely simple muting circuit isprovided whichis suit" able for use in combination with the exceedinglysimple lever and cam type of automatic tuning devices, and that suitablemeans is also provided for rendering the muting circuit ineffectiveduring presetting of the cams to permit tuning by ear during suchpresetting cf the cams to provide peak tuning at a preset position.

The disclosures of this application are illustrative and the inventionis not to be limited by them. In fact, if modifications or improvementsare not at once obvious, they may be devised in the course of time tomake additional use of the broad ideas taught and covered by thisapplication. The claims are intended to point out novel features and notto limit the invention except as may be required by prior art.

I claim:

1. A radio receiver system including in combination, a radio receivercircuit having a loud speaker therein, tuning means for said receivercircuit including a rotary shaft, a tuning member on said shaft normallystationary with respect to said shaft, but selectively adjustable withrespect thereto, means for moving said tuning member and said shaft,circuit means including said tuning member and said moving means thereinresponsive to an engagement of said moving means with said tuning memberto render said loud speaker temporarily inoperative, and contact meansin said circuit means responsive to the holding engagement between. saidmoving means and said tuning member to render said loud speakeroperative during adjustment between said shaft and tuning member whilethe latter is held stationary.

2, A radio receiver system including in combination, a radio receiverhaving a loud speaker and a circuit for feeding signals to said loudspeaker to be reproduced in sound, a tuner for said radio receiverincluding an adjustable movable tuning member, means for moving saidtuning member to a predetermined station setting, and circuit meansincluding said tuning member and said moving means therein responsive tothe engagement of said moving means and said tuning member to cause ashort circuit in the radio receiver circuit for preventing soundreproduction in the speaker, and means for opening said short circuitupon a presetting adjustment of said tuning member to permit soundreproduction in the loud speaker during such adjustment.

3. Means for temporarily preventing sound reproduction in the loudspeaker for a radio receiver circuit having a cam and lever tuner withsaid cam and said lever normally insulated from one anotherelectrically, said means including an electrical connection common tothe cam and extending to the radio receiver circuit for connection intosaid receiver circuit ahead of the loud speaker, and an electricalconnection common to the lever and extending to a corresponding point inthe radio receiver circuit ahead of the loud speaker but separated fromthe point of connection of the first mentioned electrical connection,with said cam and lever acting upon engagement there'between to providea short circuit across said two points in the receiver circuit toprevent signals from reaching said speaker to! provide soundreproduction therein.

4. In a radio receiver system, a radio receiver circuit, and a loudspeaker connected therein, means for tuning said radio receiverincluding a rotary shaft, an adjustable tuning member on said shaft,means for moving said tuning mem ber to simultaneously rotate saidshaft, and circuit means for muting said loud speaker during themovement of said member including a circuit connection from the tuningmember to the receiver circuit, and a circuit connection from.

the moving means to the radio receiver with said moving means and tuningmember normally insulated electrically from one another but acting toconnect said two circuit connections upon engagement thereof for tuning,and With said circuit connections to: the radio receiver circuit beingat such positions therein that signals in the receiver circuit areprevented from passing to said loud speaker during substantially all ofthe tuning movement of said moving means.

5. A radio receiver system including in com.- bination, a radio receivercircuit including a loud speaker, tuning means for said radio receivercomprising a movable cam, and a lever for moving said cam, with said camand said lever normally insulated electrically from one another, andmeans for muting said loud speaker during movement of said camcomprising electrical connections from each said cam and said lever tothe radio receiver circuit adapted to be connected upon physicalengagement of said cam and lever for movement of the former to provide ashort circuit across that portion of the receiver circuit intermediatesaid circuit connections thereto to prevent a signal input to the loudspeaker such as to be reproduced as sound waves.

6. A radio receiving set including a variable tuner and automatic tuningmeans for operating the tuner including a member actuated to operate thetuner and which reaches its fully actuated position only as the tuningis completed, and a muting circuit including said member therein closedby this member as the tuning begins and opened by this member as thetuning is completed, said muting circuit rendering the receiving setinoperative to reproduce the received signals.

7. A radio receiver system including in combination, a radio receivercircuit, tuning means for said receiver circuit including a rotaryshaft, a tuning member on said shaft normally stationary with respect tosaid shaft, moving means for said tuning member and said shaft normallyout of engagement therewith, a muting circuit closed by engagement ofthe moving means with said tuning member to render the receiver circuittemporarily inoperative to reproduce received signals, releasing meansoperable to release the tuning member on the shaft to permit adjustmenttherebetween, and a switch operated by the operation of said releasingmeans to render the muting circuit ineffective during such adjustmentand thereby prevent the receiver circuit from being rendered inoperativeby the muting circuit.

8. A radio receiver system including in combination, a radio receivercircuit, tuning means for said receiver circuit including a rotaryshaft, a tuning member on said shaft normally stationary with respect tosaid shaft, moving means for said tuning member and said shaft normallyout of engagement therewith, a muting circuit including said tuningmember and said moving means closed by engagement of the moving meanswith said tuning member to render the receiver circuit temporarilyinoperative to reproduce received signals, and a demuting switch in saidmuting circuit for rendering the muting circuit ineffective even whilesaid moving means is in engagement with said tuning member.

9. A radio receiver system including in combination, a radio receivercircuit, tuning means for said receiver circuit including a rotaryshaft, a tuning member on said shaft normally stationary with respect tosaid shaft, moving means for said tuning member and said shaft normallyout of engagement therewith, a muting circuit including said tuningmember and said moving means closed by engagement of the moving meanswith said tuning member to render the receiver circuit temporarilyinoperative to reproduce the received signals, and a demuting switch insaid muting circuit for rendering the muting circuit ineffective evenwhile said moving means is in engagement with said tuning member, saiddemuting switch being operated by said moving means at the end of itsmoving stroke.

10. A radio receiving system including a receiving circuit adapted to betuned to different frequencies by means of a tuning shaft, a pluralityof cams on the shaft normally lockedon the shaft to rotate therewith butreleasable at will for adjustment with respect to the shaft, a tuninglever for each cam adapted to be actuated to rotate the cam to apredetermined position whereby the receiving circuit may be tunedautomatically to any one of a plurality of frequencies by selectiveactuation of the levers, a muting circuit for rendering the receivingcircuit inoperative to reproduce signals during such automatic tuningconnected through the actuating lever and the cam whereby the circuit isclosed by the en-.

gagement of the lever on the cam, and a normally closed switch in themuting circuit including a bar positioned to be engaged for opening theswitch by each of the actuating levers as it reaches the end of itsstroke.

11. A radio receiver including in combination a radio receiver circuit,means for tuning said radio receiver circuit including a rotatable shaftand a movable control member thereon, an actuator for moving saidcontrol member, friction varying means acting on said control member toprevent or permit angular adjustment of said control member withreference to said shaft depending upon whether friction is applied to orremoved therefrom, a muting circuit including said control member andsaid actuator therein and connected into said receiver circuit andincluding a demuting switch, with said switch being mounted in aposition with reference to said friction varying means whereby saidswitch is open to continuously maintain said muting circuit open whenthe friction is substantially removed from said control member andclosed to maintain said muting circuit closeable when the friction isapplied to said control member, with said muting circuit adapted to beclosed when the demuting switch is closed upon engagement of saidcontrol member and said actuator.

12. A radio receiver including in combination a radio receiver circuit,means for tuning said radio receiver circuit including a rotatable shaftand a movable control member thereon, an actuator for moving saidcontrol member, friction varying means acting on said control memher toprevent or permit angular adjustment of said control member withreference to said shaft depending upon whether friction is applied to orremoved therefrom, said friction varying means including a knob unit onsaid shaft having threadable axially adjustable means therein forcausing axial movement of said knob unit, a muting circuit includingsaid control member and said actuator therein and connected into saidreceiver circuit, said muting circuit including a demuting switchmounted in a position with respect to said knob unit whereby said switchis open to continuously maintain said muting circuit open when thefriction is substantially removed from said control member and saidswitch is closed to maintain said muting circuit closeable when thefriction is applied to said control member, with said muting circuitadapted to be closed when the demuting switch is closed upon engagementof said control member and said actuator.

13. A radio receiver including in combination a radio receiver circuit,means for tuning said radio receiver circuit including a rotatable shaftand a movable control member thereon, an actuator for moving saidcontrol member, friction varying means acting on said control member toprevent or permit angular adjustment of said control member withreference to said shaft depending upon whether friction is applied to orremoved therefrom, said friction varying means including an internallythreaded toothed unit vertically carried on said shaft and operativelyconnected with said control member, and a pivoted lever for engagingsaid teeth of said internally threaded toothed unit for maintaining saidunit fixed while rotating said shaft with reference thereto, a mutingcircuit including said control member and said actuator therein andconnected into said receiver circuit, said muting circuit including ademuting switch mounted in a position in respect to said lever wherebysaid switch is open to continuously maintain said muting circuit openwhen said lever is in engagement with said teeth and said switch isclosed to maintain said muting circuit closeable when said lever is outof engagement with said teeth, with said muting circuit adapted to beclosed when the demuting switch is closed upon engagement of saidcontrol member and said actuator.

WILLIAM L. DUNN.

